Hydraulic fracturing is a term that has been applied to a variety of methods used to stimulate the production of fluids such as oil, natural gas, from subterranean formations. In hydraulic fracturing, a fracturing fluid, usually an aqueous fluid, is injected through a well bore and against the face of the formation at a pressure and flow rate at least sufficient to overcome the overburden pressure and to initiate and/or extend a fracture(s) into the formation. The fracturing fluid usually carries a proppant such as sand, bauxite, glass beads, etc., suspended in the fracturing fluid and transported into a fracture. The proppant keeps the formation from closing back down upon itself when the pressure is released. The proppant filled fractures provide permeable channels through which the formation fluids can flow to the well bore and thereafter be withdrawn.
In hydraulic fracturing, proppant particles under high closure stress tend to fragment and disintegrate. For example, at closure stresses above about 5000 psi (34,500 kPa), silica sand, the most common proppant, is not normally employed due to its propensity to disintegrate. The resulting fines from this disintegration migrate and plug the interstitial flow passages in the propped formation. These migratory fines drastically reduce the permeability of the propped fracture.
Other propping agents have been used in an attempt to address this problem. Organic materials, such as the shells of walnuts, coconuts and pecans have been used with some success. These organic materials are deformed rather than crushed when a fracture closes under the overburden load. Aluminum propping agents are another type of propping agent that deform rather than fail under loading. While propping agents such as these avoid the problem of creating fines, they suffer the infirmity of allowing the propped fracture to close as the proppant is squeezed flatter and flatter with time. In addition, as these particles are squeezed flat the spaces between the particles grow smaller. This combination of decreased fracture width and decreased space between the particles results in reduced flow capacities.
Another type of proppant includes spherical pellets of high strength glass. These high strength glass proppants are vitreous, rigid and have a high compressive strength which allows them to withstand overburden pressures of moderate magnitude. In addition, their uniform spherical shape aids in placing the particles and providing maximum flow through the fracture. While these beads have a high strength when employed in monolayers, they are less satisfactory in multilayer packs. In brine at 250° F. (121° C.), the high strength glass beads have a tendency to disintegrate at stress levels between 5000 psi (34,500 kPa) and 6000 psi (41,400 kPa) with a resultant permeability which is no better, if not worse than sand under comparable conditions.
Resin coated particles have been used in efforts to improve the stability of proppants at high closure stresses. For example, U.S. Pat. No. 3,492,147 describes proppants useful in fracturing operations in which the proppants are produced by coating a particulate solid with an infusible resin. The particulates to be coated include sand, nut shells, glass beads and aluminum pellets, whereas the resins used include urea-aldehyde resins, phenol-aldehyde resins, epoxy resins, furfuryl alcohol resins and polyester or alkyd resins.
Although resin coated particles have proven satisfactory in numerous applications, concern exists over their use under high closure stresses. For example, some self-consolidating, resin-coated particles of the prior art do not develop their full strength until the resin coating has cured in the formation. In the event of rapid closure of the fracture, the proppant could be crushed before the resin has cured, resulting in decreased permeability. The use of dual resin coated particles as described in U.S. Pat. No. 4,585,064 has therefore been proposed as a way to alleviate this problem. In particular, in the '064 patent the proppant substrate is provided with an inner coating of a substantially cured resin to increase the crush resistance of said substrate and an outer coating of a fusible curable resin which fuses and cures when injected into a formation to create a cohesive permeable mass.
One concern with the use of self-consolidating, resin-coated particles is compatibility with the well treatment fluids used to transport such particles into a formation. To address this concern, U.S. Pat. No. 5,837,656 discloses a dual resin coated proppant that combines the well treatment fluid compatibility advantages of precured resin coated particles with the strength and resistance to migration of self-consolidating proppants. The proppant comprises a particulate substrate coated with an inner coating of a fusible curable resin; and an outer coating of a substantially heat-cured resin, where the resin of the inner coating is selected from the group consisting of phenol-aldehyde resins, urea-aldehyde resins, melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins, and copolymers of such resins; the resin of the outer coating is selected from the group consisting of phenol-aldehyde resins, urea-aldehyde resins, melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins, and copolymers of such resins, and the resin of the outer coating is heat-curable at conditions that leave the resin of the inner coating uncured.
Although resin coated particles offer significant advantages as proppants for well treatment fluids, the resins currently employed are generally derived from petroleum making them subject to the same supply constraints and price increases as their base raw material. In addition, many of the current resins, such as the phenolic resins, contain impurities, such as free phenol and/or free formaldehyde, which can negatively interact with the fracturing fluid used to suspend the coated proppant as it is being pumped into a formation. Furthermore, it has been shown that non-reacted hexamethylenetetramine, which is commonly used to cure or partially cure the novolac phenolics, can also leach out and negatively impact the fracturing fluids as well.
U.S. Patent Application Publication No. 2008/0202750, published Aug. 28, 2008, discloses thermoplastic coated proppants. These thermoplastic coated proppants are both free-flowing and not tacky at ambient conditions. However, at elevated temperatures and pressures often encountered in subterranean formations, the coated proppants exhibit latent tackiness which results in the agglomeration of the coated proppants to form a stable framework of agglomerated proppant particles. Such a stable framework or network of agglomerated proppant particles reduces both solid particle flow-back and the transport of formation fines from the subterranean formation. Examples of thermoplastic materials, which may be used to coat proppants, include polyethylene, a polypropylene, an ethylene vinyl acetate, an ethylene ethyl acrylate, a styrene-isoprene-styrene, an acrylonitrile-butadiene-styrene, a styrene-butadiene-styrene, a polystyrene, a polyurethane, an acrylic polymer, a polyvinyl chloride, a fluoroplastic, a polysulfide, a styrene-acrylonitrile, a nylon, a phenol-formaldehyde novolac resin, or any combination thereof. In another aspect, the thermoplastic material is a pine rosin, a modified rosin, a rosin ester, or any combination thereof. Further examples of such thermoplastic materials include a terpene resin, a coumarone-indene resin, an oligomer of C5 hydrocarbons, an oligomer of C9 hydrocarbons, an oligomeric reaction product of a terpene and a phenolic, an oligomeric reaction product of a terpene and a styrenic, or combinations thereof. Generally, the number-average molecular weight of these oligomeric materials is less than about 10,000, and more often, less than about 5000. The number-average molecular weight of the terpene resin, the coumarone-indene resin, the oligomer of C5 hydrocarbons, the oligomer of C9 hydrocarbons, the oligomeric reaction product of a terpene and a phenolic, and the oligomeric reaction product of a terpene and a styrenic, may be within a range from about 100 to about 4000. The number-average molecular weight of these materials may be in a range from about 125 to about 3000, from about 150 to about 2000, or from about 200 to about 1000.
According to the present invention, it has now been found that a resin system obtained as a product of the Maillard reaction between a carbohydrate and an amine or an ammonium compound provides an effective and advantageous coating for proppant particles. This invention may be used as a single layer to enhance crush resistance and especially when used as the outer cured coating of multi-coated proppant particles, such as described in U.S. Pat. No. 5,837,656. Thus, not only is the resin system derived from renewable biological resources, but also the system does not contain the free phenol and/or free formaldehyde that can degrade many fracturing fluids.
U.S. Patent Application Publication No. 2007/0027283, published Feb. 1, 2007, discloses a binder, comprising: Maillard reactants including (i) an amine and (ii) a carbohydrate, wherein the binder is (i) uncured and (ii) formaldehyde free. However, the binder is used to fabricate materials from non or loosely assembled matter, such as glass or cellulose fibers. A similar binder for wood particles is disclosed in International Patent Publication No. WO 2008089847.